Modern Steel Products (2014) - Formable steels: lecture 19

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Modern Steel Products (2014) - Formable steels: lecture 19
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19 (2014)
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Professor de Cooman talks about formable steels. This is a part of a course of lectures given at the Graduate Institute of Ferrous Technology, POSTECH, Republic of Korea.
The Graduate Institute of Ferrous Technology (GIFT)
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the of this thing we'll be talking about former Polaroid carbon steels some of which are mainly produced combination of hot strip mill in Colstrip Maryland and nearly depending won't have the process these materials are process when we generally divide them 2 groups on the aluminum killed low-carbon steels and then interstitial about 40 steals then we also have will discuss this later on a number of high-strength steels which which is usually referred to as advanced high-strength steels which slightly more complicated Mike rastructure and will say few words about this class of steals the peace deal trips steals and complex phase steals but will 1st discuss the aluminum killed low-carbon steels and EIS meals and that these are processed as I said the hot rolling and cold rolling and they're basically I'm slight differences depending on how you Our whether you eventually a lot you continues annealed and so the biased use could also be that yield continues in yield very often but certainly in In applications for instance such as automotive applications on these steels and up being galvanized yes and so if you betcha meal than they usually end up being electoral galvanized them the end of the continuous annealing that you would use if the product has to be galvanized you would apply pocket galvanizing instead of the continuous galvanizing and which seemed why that is that is because In the called the galvanizing like you can do 2 things at the same time we crystallization and kneeling on the coating application in the case of these what we call advanced high-strength steels this a little bit of a challenge here is difficult to make these steels because of the complex Mike rastructure as in a batch annealing furnace so you basically require continuous annealing processing of these grades too actually makes the required Microsoft and that's the reason why we call them advanced high-strength steel the Basic Law you will see that the basic grades aluminum killed a low-carbon and I have steals or ovaries soft steals and that we will use methods To increase there their strength Nancy the strength increases quantities the the cost of this group steals this far high-strength steels and that this the method the methods we will use to increase the strength will include solid solution hardening with Of course manganese silicon but also phosphorus that grain size reduction of the grain size will be the main ways in which we do the strengthening of increases the strength of these 5 of the steals will see and then have there is also the group of steels which recall bake hardening steals the Cardinals steel lobby page steals and where we use there static strain aging step extreme images 2 I want to do just the hardening you have to let served it's a short overview here let's go through the material In his letter material some of it are a repeats of what you've already seen so will go relatively quick because the concepts now I assume you know them well right so that
important here is that that to for its when he steals applied vary widely applied in construction of a cars for instance so the important here is to realize that parts for automotive applications are usually divided into visible parts and parts that are not too visible we talk will will call them structural parts or in their pods and because they tend to be large and flapped wheels often call them panels interim panels and outer petals other panels I have extra demands in terms of visual appearance against In contrast to in panels where strength aspects may be more important there the rights of this is this is 1 thing you have to give them up and then we also have the number of parts of their again not visible unless you you can't use the car up like this 1 which of very high strength such as France's these impact beams in at the doors of passenger cars in but which which can be made from a the ultra high-strength steels the which strengths beyond the Giga Pascal OK so very
important with the steals that we applies that they shouldn't strain hardening so we don't want any yield .period illuminations there's 1 customer in the the car maker uses them the other thing we
want is to have a high Our value and we know but this is related both to Of the crystallographic texture and the composition so if we have as if I want to have a high R M value mean value of 50 I need to have a very large the use of motor this with X-rays intensity of 1 1 1 reflections yes but the which means they have a very strong and very high 1
1 1 parallel to normal direction fiber and I also need to have a low carbon content and low carbon content has the preferably if you can if you see here is that these are value will be high at around or less then 10 the PPS so you you you want to either you steal stuff have the vacuum D Gaston where you have a very very low carbon contents and where you buy into the the carbon with fighter Itanium on the form of fighting you on Iulian precipitates or you precipitated affected efficiently passed a Carbide and that's what we do In the aluminum killed low-carbon steels OK so
a few things here about the the processing of these the steel the 1st of all we have important parameters In the hot rolling In the cold rolling and the annealing yes and and whether these parameters Welt the finishing rolling temperature and the coiling temperature so here we're talking about the aluminum killed low-carbon steels OK so but the form ability depends on the finishing temperature is we finish rolling in the heart of the strip mill at temperatures above the 8th 3 temperature so where we have some gamma face President yes but we stop seeing an increase in the FAA value so rolling in the two-phase region or in the Austinite region is not a good idea 2nd but the coiling temperature here we see that the pending on the coiling temperature and we could have a high are values or very little our values and we know that this is related to the precipitation of aluminum nitrite for when we do In the case of a batch and yield Gold struck so if you have a low 1 the little calling temperature the aluminum nitrite it's in solution and that remember that is what you want to have aluminum nitrite is in solution this institution the rest of if you call at high temperature the aluminum nitrate precipitates and doesn't give you the the texture of the tour in case of the cold rolling and annealing again what's important is the amount of cold rolling hills the debate peak value of the R parameter of the land for value increases endit peaks at around Little over 70 per cent of formation of low-carbon steel grades don't gain in when you deformed them Beyond 70 per cent of the that's the important thing to know and when you when you are the deciding for instance the hot strip thickness but the other thing is the annealing temperature the annealing temperature remember that we discussed the Bachand healing and that the temperatures there were of the order of 700 deg C so their the A temperature why is that if we go beyond the 81 temperature we introduced home Austinite into microscopic that has an impact negative impact of the texture development and we see began a reduction in the form of the list what is important to remember whether these deals is they have very simple Mike rastructure yes so that the properties that we are mechanical properties that we have to the yield strength tensile strength and of the the longer Asian or basically 1 of controlled by really a few parameters 1st of all have very important also as always is the grain size but and we have a whole batch relation for the yield strength we have a whole page the equation for the tensile strength and the elongate is pretty much independent of the eye of the grain size I think would discuss is that there will be a slight decrease that there will be there will be a decrease of course that's when you reduce the grain size very heavily in the case of of you know technical situations uh grain size you can't get much lower than that of 10 15 microns anyway so for all practical purposes the longer Asian does not degree then but we add that we can have strengthening elements such as manganese silicon phosphorus In general what we see is that of course they will increase the tensile strength the yield strength and tensile strength and the will a decrease in the elongating Sherman and now and then we have to temper rolling that is another important thing because we always have to do it now and in particular for the Lunar killed low-carbon steels which always contain relatively high amounts of carbon but you have to do the temporal linked to get rid of the yield point and the yield pointing to communication and so as you the amount of temporal length has will decrease the the yield strength of course you will also have a better and a decrease in the elongate should up but to the tensile strength that this remains unaffected OK
so again important and then we will discuss this in the past is that is how you process the steel that if you process them in these animals killed carbon steels low-carbon steel if you process them in a batch kneeling situation and in the continuous in dealing with a situation you have very different situations in terms of carbon and solution In the case of the batch annealing you have near equilibrium conditions in the case of the continuous annealing you have to have an overreaching steps to precipitate as much Carbide's as you can
we just so let's say the spent see typical ranges of elements important elements in the seals aluminum killed low-carbon steel is very very very lean compositions yes very lean composition so where the carbon contents will typically be in the range of 200 to 500 ppm the nitrogen content will it is smaller than 100 but really the number you have to remember is typically 48 PPS yes and then we have very important is the aluminum additions and their 200 to 500 again typically is up about the midrange here 300 ppm and then the manganese content is 1500 to 2000 ppm but is a ppm all of these so that's 0 . 1 5 manganese writes very long and that and so this is really the composition for the most important Bachchan meals aluminum killed low-carbon steels very lean compositions OK even
so get what happens With the aluminum nitrite is very important in the States for 4 reasons of course is that you want to eventually buying the aluminum to nitrogen as well but in the case of the Bachand Nielsen route it's also a way to the 2 things is to get good texture and at the same time buying the nitrogen OK so what do we see when we look at the solubility of aluminum nitrite In Boston what do we see if we have about 100 ppm of nitrogen and about here 370 ppm of aluminum that is a typical of the composition of this is the composition of and this is the solubility life and so you know that if I have a composition that's above the solubility line I should have this is a this is the solubility line at their 918 so that is in the Austinite the conditions in the Austinite stability range so what should happen is the nitrogen should by-and-large precipitate as aluminum nitrite yes so aluminum nitrate should form in the Austinite and it's illustrated here but it doesn't it does not work well it does not this is the kind that take climatically the difficulty in if you look now because this is an equilibrium diagram right but and it is whether or not something happened and how fast it happens you get to see if you plot as you know PPT DI pp T diagrams right the 2 principal to PTT the precipitation time temperature diagram so and see for instance if I'm here at this temperature 980 here yes does the and uh precipitation of the aluminum nitrite in Austin nights it will take for a very long time over 100 minutes so it basically does not occur In the Boston however when you as soon as you make far-right yes I am the precipitation starts you have very old and very effective precipitation of the aluminum in the India and fair and but what is important here is of course is the choice of your temperature if your temperature if you choose to coil that's 7 hungered or 600 years you will have to extend this you will have enough time in the corner old material and remember that the coiled material cools down slowly the cooling arrange this the 20 degrees Brouwer has so and you will have plenty of time to pursue particularly in light in the fair is however a few coil below say 500 yes 50 there again be the year of the growth of the aluminum nitrate stops
because you don't have aluminum diffusion but the a choice of the coiling temperature does not only influence the fight whether or not aluminum nitrite this it also has an impact on the the Carbide's that you for that normally if you have 200 to 500 ppm carbon so this is here it's 7 look at this phase diagram here so this has to this point here is 200 And this point here is 500 yes ppm of carbon what does that mean what does that tell you that the tells you that normally you should have perlite into might restrict when their own however whether or not you have pro-life and how the perlite will look like on the same entitled look like depends very much on again on the temperature at which should do the calling I'm so let's let's say of the discussed this for a moment so in so if again we're talking about aluminum killed low-carbon steels but so in the hot strip mill you have you reheating the finishing a cooling in the quarreling has spent so you have the temperature as a function of position and the grass here are a 4 at 300 ppm carbon low-carbon steel which .period 15 manganese right so if you want wonder where these curves come from not so you do that the deformation the rough rolling in the Allstate stability rings you the finishing in the arsenic and then you cool down and you know in and runout table here you to the transformation of the transformation very quick like this distills a found out you can see here this is this is your TGT diagram right the transfer the the formation of this instead very quick I and set so you you start doing you start getting the transformation here yes but and as and and as you coil yes he did a very little cooling rates has so the it depends now on how deep you have been cooling yes princess if you go but at 700 deg C yes you will have transformed the you have formed a pro you tech toys variety but you going to cool down and for you summon tied in conditions of very very low and very slow local yes so In this case you may not even form any perlite the and what you you will get is what we called the forest the statement on the mindset that the Symantec's absolutely not connected to business forms absolutely no pursuant those who were like him or you can coiled deeper yes France's 600 law and this means that the perlite transformation is the former you do the coyly and in this case I do perlite transformation after I'm calling in here do it before I call yes so the distribution of the same entitled The essential and that and and again also as I said if you coil at high temperature you will precipitate the aluminum nitrite if you coil at low temperatures you will not precipitated the aluminum nitrate but right and this
is and this is a calculation here
to show you what happens to aluminum nitrite so is basically so this line here is the temperature when I'm going down in temperature absolutely and I'm I'm looking at tracking the radius of aluminum nitrite particles so what is interesting here is that of the temperature goes from around 6 15 2 520 and what you see here is that the in the far the aluminum nitrate particles grow very quickly but then the growth stops growth stops at about that about this temperature and what is the temperature here temperature is below 600 guests below 6 and as a rule of thumb you can assume that when you close to 550 all the substitution all elements basically stopped diffusing yes and if you want to precipitate a particle a Carbide and I tried what have you you need to particles 2 elements need need to come together b aluminum and the nitrogen from nitrogen can diffuse easily at 550 but aluminum stops diffuser so the particle the aluminum nitrate cannot grow anymore because the aluminum doesn't diffused to the particle yes and that's the reason why the but the the use of the particle stops growing the aluminum In solution is now flat and then of course there will be if I don't precipitate all the nitrogen there will be some nitrogen left In solution and so so the you hear the says it's a little bit lower than what I said at 5 20 you effectively don't have any
the aluminum of the nitrite is so but it also has a big impact on the on the properties of your material in terms of the properties in their length Of the Court and safe you this is a hot rolled coil years and you have to wraps and interacts Of this court and the under wraps and the interrupts Will it be exposed to higher cooling rates always and as a consequence 2 things happen it is that of the carbide distribution is different on these ends then the aluminum nitrate precipitation is different at TX let's have a look at the precipitation of the nitrogen as the nite trying to as aluminum nitrate if we call oil at high temperatures well what we see is that we precipitate most of it in the center of the coil but at the on the wraps and the outer wraps we don't why is that well because the interacts with the the coiling we're quarreling on the Mandarin the demand role of the courts and the Mandrell is cool that distract may be arriving at around 700 deg C but when it touches the demand role it'll be cool but it'll be it'll be much cooler and so the cooling rates will be faster and and I'm not going to precipitate any aluminum nitrate yet and the same at tea on the exterior wraps there on the exterior so that they cool down much faster then the interior so my product will be we will have in homogeneous properties yes and if you have a demanding customer it it means that you have to scrap these ends this or figure out a way to solve the problem 1 of the ways yeah it's solved in practice is by having a slightly warmer bands all this you instead of calling the start the head and detail of your a lot as you would to the middle you reduce the cooling them so that they they're hotter 1 day and so you can compensate for this but of course if I do if I had done the coiling at low temperature yes then then did discuss a cooling rate doesn't really matter and I have very little nitrogen aluminum nitrite precipitated yes and the other thing what is very nice and you can see this I already here this if I have a strong nitrite Binder like Titaniums Itanium Virginia nitrite forms Titaniums nitrite Itanium forms stating 900 at high temperatures already so there's never any problems for its precipitation has to deal with the United and is fully precipitated by the time you were calling the structure yes that's for the nite drives what about the the year and the Carbide's well you can see the difference although the sensitivity 2 the the higher cooling rates at 2 tail end and that the uh head 2 of the 8 stretch out if you make a deal strength knows when because you have higher calling rates again is for a whole bandits "quotation mark and 720 at the head and that detail yes you see highest-ranked higher yields struts and the reason is because get fired carbide In the microscope to him so but that is you can already see that that will be used in home watching the piece in the strapped Lang's work it will be an essential a problem with aluminum killed low-carbon steels in certain cases for certain process because obviously if you if you were using a little coiling temperatures of these effects would be minimized but then it means that you would have to go into batch annealing and because the batch kneeling you'd must have a low calling temperature coiling temperature must have been lot on the
menu and and the reason is shown here and I just want to remember remind you of the fact that there is an optimal excuse me heating rates in the mean Bachand healing process where the recruit's stabilization Of the mike rastructure cold-related structure we crystallization climactic intersects With the aluminum nitrite precipitous and gives you this pancake microfracture and you know that has a very high form ability and so and to achieve this of course you need to have your aluminum
nitrate hints of solution 10 people have studied this process by and at what they see is that the number of grains with the right fiber orientation will increase strongly during the the annealing and it's during this very slow annealing alright so on again this year the this former ability parameter and our values Is there Strong function of the role OK because it's the bowling that gives you the texture of them and the maximum here for aluminum killed steals is around 70 degrees as 70 per cent and the minimum is around 50 per cent so because if you if you are if you have less than 50 per cent of the formation you will get very sluggish repressed life which the so you basically want to work around this point yes but it also means that if see if you if you need to make 1 millimeter thick the strapped for your client yes that means that the the way you can control the deformation here the amount of the information is by choosing the right hot strip thickness it's that you don't have much choice because you need to have a 70 per cent of defamation you can already see that would I have steals the dependence gives you a lot more possibilities here you can increase the thickness it's a the are value bye increasing the the reduction beyond 80 per cent so it also means that you can process thicker and thicker gage hot strip yes that's in terms of productivity deals are also interesting what other
problems do we have with no room for challenges to have with carbon steels well it turns out that the carbon gets in the way the texture formation this we I went when we have free carbon in the Microsoft drudgery we have we have the problem of texture formation and and we know how to solve this we basically need to have the carbon it has to be precipitated yet but there is a little patch yes is that carbon and manganese attractive interaction yes is so when you manganese 2 a low-carbon steel the precipitation of semen type will be delayed because because of this attractive interaction between carbon and manganese yes the manganese will be kind of kept in solution by the presence of manganese we call this this combination of substitution or following element and interstitial elements that we call these dipole it's because I these elements like to stick together that's so as a consequence what we usually what we see Is that we are value it is higher when we reduced the carbon content from 4 400 the 70 ppm to 10 ppm as it increases yes but there is a negative impact from the manganese content so when the manganese content increases yes I see a decrease In the aura value because this problem the and and you can see it remember and the impact of the manganese is the most strongly felt and so the typical low-carbon steels and will have as I remember 200 2 500 ppm of carbon and so that's typically in this range show that this will be his is correct the view of things here for the dependence on the manganese content and you can see here the the lowering decrease is exactly inside the typical manganese range that you have and people have studied this in detail uh and you you it's actually In order to get the best combination of properties uh did considering that you have about 200 to 400 ppm of carbon this is the actual range where you can achieve the best possible form ability again here I want you to know that In I steals yes it is a matter what manganese content you have this the the R value is high because I don't have a carbon in solution yes and and and it's irrelevant what the manganese does To the carbon in this stage in this great because there is basically no carbon
solution yes right and we talked about to do the
temperamental so again this season Our value as a function of heating rates basically saying what is the the effect of housing indeed aluminum killed steel process With mole processed with local heating temperature so if you have no coiling temperatures it's it's best to process this steel during the annealing re crystallization deal with low heating rates because that's where you get the highest are value if you have coil date at a new high temperature you want to have fast heating rates you don't need to get slow heating rates but you see that the best but the best are values are obtained In Bachchan healing and so low heating rates and for the material that's corralled at low temperatures and that's why there are some companies who for many years have not used continuous annealing technologies because they didn't feel it was worth the investment yes because you you can get pretty good by form ability if you use the batch wrote
but let me know skipped this
because it's a little bit too technical
grain size is interesting points and I'm not going talk to much about this In general you would think that grain size is a you know you'd like to have a small grants based but in the end the case of texture development actually having a slight grain growth is advantages because the grains that grow yes tend to be the ones that have the right texture yes the larger grains means less grains that don't have the right orientation so but what you see an increase in the the you are vital in the form of ability having said this is this something you really want to know the exploits a lot not really because of obvious leader of the decrease in strength has found but the other problem is that Is the following when you have a large grains yes I need to form a panel yes the grains with them usually when you the former grains it's it doesn't remained flat it starts to there the the become waving the grain becomes waving him now that's not too bad if you have small grains that's not very visible once the grain start to be large it gives what's called orange peel effect as and basically it's it's like you have a higher roughness if it the Swedish surface becomes rough and that is usually not acceptable when you have Italy a painted panels because it has an impact on the quality of the visual appearance of that and in particular for carmakers this is really important 2 doesn't have the isn't really have a huge impact on strength or anything but it has a huge impact on the visual appearance of painted panels and himself and that that's cold orange peel orange peels usually related to large groups of got
so now no so we we've talked about the important things In the case of low-carbon steels you of carbon steel and I want to remind you Of the fact that so what's important here is the aluminum nitrate the human tied this human tide precipitation texture and the aging of the snow no you .period and Of course I I also want to remind you of the fact that when you precipitating aluminum nitrate you also get rid of nitrogen and solution and and so on you don't dread of the but the strain aging that may be caused by United now let's look at our the I steals last year at the I steels are characterized by 2 important differences if you compare them with the low-carbon steel in terms of composition 1st of all there vacuum treated vacuum trade means I have low-carb right not 200 to 500 ppm but what this is this is the typical Very Large Array tend to 80 Tampa Coley yes I know steelmakers can go easily less than 10 but there is some pick-up and and things like this so usually usually less than 20 ppm yes In the nitrogen is pretty much the same around 40 ppm but what's really important here silicon and the manganese is comparable phosphorus sulfurous comparable aluminum is comparable don't forget we always need to add aluminum to kill the steel yesterday due to remove the oxygen and that we have excess from from the steelmaking but you still have aluminum in steel but what you add ice Titaniums or mail opium to their own different ideas still to have steel which where you only at Kai Tania and steals where you at hiding him and and went to these they they stabilize the the steel state they is stabilize the carbon and nitrogen just a few words here about these areas still than of composition so you have Titaniums I steals which we write like this or Titane you you'll be empire of steel if this is so widely different concept In the composition here when you make Titaniums I steals the Tania will form nitrites it will form Koble sulfides it will form the sulfides and it will form tightening cars so it takes care of the nitrogen and it takes care of the car and if you can for Kabul's sulfide of Titaniums it also takes care of the house of cards will talk about in the case of a Titaniums albeit Titaniums form stating you might try an ideal niobium forms niobium carbide and that's it why would you use 1 and not the other well usually With Titaniums I have steel we haven't and we have to add an excess of tightening we had 100 heard about 100 ppm of excess pointing you in order to make sure that that Union Carbide is for we need to have an excess of hiding so we add more than is needed From this documentary point of view to stabilize the card and it turns out that this can give surface the facts surface defects In the galvanized galvanized products yes you can give Titaniums related typical this would be oxides yes and so on and so you you don't want this so we we use citing United albeit against where we don't have a problem of having to add in excess of 20 new or in excess of 90 opium yes you would think that that has this is positive for for the calls right
and tensile properties here into the use can be very very soft materials yes hundreds of 40 280 yes but if you don't temporal these materials here can be as low as 120 has really really soft very large the longer Asians yes Hi values very low delta our values so these materials are absolutely perfect just for form ability difficult form ability of situations we are not going to
go into this is very much the box I do want to tell you that the formation of these Carbide's and nitrites at at 1st looks rather simple it's a bit more complicated than this yes and this is illustrated here the reason why it's a little bit complicated it's because of this and the fact that tight Tania conform sulfites and cobbles sulfides so if if you look at the precipitation sequence also these various Titaniums precipitates during hot rolling In the hot rolling process it's a bit complicated the 1st year the easy thing is to die that nite and nitrogen bind that have a high temperature but then this is followed by around 13 12 hundred here by sulfide formation endit if they temperature and so remember 1200 that's the reheating furnace if you're reheating furnace temperature is a little bit lower you can Florida Carbo sulfide now this is an interesting precipitous because it's the only precipitated that for that binds carbon at high temperature yes so and sold myself and other people have also tried to develop steals based on this concept because if you if you have something that combines carbine at high temperature you can easily get rid of cards this way and very efficiently however you need to reduce the reheating temperature as to get this precipitation go so and that's that's an issue in terms of productivity and as a temperature decreases you start forming you Titaniums Carver In the and it's not finished in the if if you have for instance added some small amount of phosphorus because this is a very soft it it becomes interesting to add small amounts of phosphorus to the steals to strengthen them less than what you see is that in batch annealed by U.S. steals the phosphorus can react with the Titaniums Carbide and the car was sulfites to form Byron Titaniums falsified tests so making the the whole situation is a bit more complex but I want to work close at this point with the sight of still continue their next Tuesday the endure With wit to tighten your students


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